1. Field of the Invention
The present invention relates generally to liquid crystal displays and more particularly to a method for bonding together liquid crystal cells using a transparent, highly viscous non-curable grease.
2. Brief History of the Art
Many classifications of liquid crystal displays incorporate multiple liquid crystal devices in their fabrication. For example, one such type of device, a double super twist display, is constructed using an active LCD cell and a compensating LCD cell. Another such type of device, a liquid crystal shutter display, is normally constructed using two .pi.-cells, and, in some applications, a third anti-reflection sheet of glass.
When fabricating these types of displays, it is often desirable to avoid air gaps between the optical elements. Air gaps tend to cause internal reflections and transmission losses which ultimately reduce output image quality. Thus, for these displays, it is highly desirable to place a transparent material between the liquid crystal cells that is matched to the indices of refraction of the bordering optical elements, ie., the inter-lying polarizing films or glass substrates. The addition of this material effectively eliminates any air gaps and minimizes the internal reflections and transmission losses.
Prevalent contemporary solutions in manufacturing either double super twist nematic liquid crystal or liquid crystal shutter displays which do not leave air gaps between individual cells include applying permanently bonding gels or epoxies between the cells which harden as they cure. Two-part gels and epoxies are commonly used and offer the advantage of permanently sealing the optical elements together, however, their use also presents several disadvantages.
First, the initial viscosities of these materials prior to cure must be sufficiently low in order to allow good mixing. As a result of this limiting low viscosity, the component parts of the gels or epoxies can readily flow over unwanted surfaces before they have time to solidify. This problem can be avoided if the liquid crystal cells are placed into a mold capable of containing the component parts during the curing process. However, a reusable mold is an inadequate solution because it must be able to release the bonded assembly after the cure without needing to be cleaned. A cleaning procedure after each casting would be prohibitively expensive in a high volume manufacturing process. Additionally, a mold which becomes a permanent part of the display assembly, while eliminating the cleaning problem, still presents added cost and design difficulties.
Additional solutions to optical cell coupling include ultra-violet cured materials and plastic films. Although ultra-violet cured materials offer many theoretical advantages, the transmission properties of the sheet glass and polarizing films as well as the stability of the liquid crystal materials to ultra-violet exposure all prohibit this alternative. Plastic films matching the x-y dimensions of two rectangular liquid cells can function well as the optical coupling material, but both sides of the film must include an adhesive glue layer that binds to the surfaces of the glass and polarizers. These films are prohibitively expensive and difficult to apply without image-deteriorating air bubble defects.